FUELS (GENERAL METHODS) 305
of the maximal temperature. This correction is easily made by means of
the thermometer readings before ignition and after the maximum, these
giving the mean thermometric variations per minute in the preliminary
and final periods. It is then assumed that, during every minute of the
period between ignition and the attainment of the maximum temperature
the temperature varies uniformly, so that the correction may be referred
to the mean temperature of the minute considered. If the mean tempera-
ture of a definite minute differs by less than ±° from the maximum, it is
held that the diminution of temperature due to loss of heat during that
minute is equal to the mean diminution in every minute after the maximal
temperature j if, however, the mean temperature of any minute differs
from the maximum by more than i° and less than 2°, the correction for
that minute is taken as the mean diminution after the maximum tempera-
ture, decreased by 0-005°. Finally, for the first half-minute after ignition
it is assumed that the variation is equal to the mean observed before the
ignition.
Besides this correction, account must also be taken of (i) the heat of
combustion of the iron coil, 1-6 cal. being allowed per milligram of iron, and
(2) the heat of formation of the nitric acid, which is 0-23 cal. per milligram
of nitric acid (also of the heat of formation of sulphuric acid'; in the open
air, sulphur dioxide will be formed and the correction to be subtracted is
2-25 cals. per milligram of sulphur or 0-73 cal. per milligram of sulphuric
acid).
The calorific power p (in the case where the sulphuric acid has been
calculated as nitric acid) is, therefore, expressed by the following formula:
p = (Ti-T + t}(A + a] _o-23« -i-6/ . . . fi),
where,
T = observed temperature before ignition,
T1 = maximum temperature,
t = correction for heat given up to surrounding air,
A = weight of water in the calorimeter,
a = water-equivalent of the apparatus, this being determined once and
for all in a preliminary experiment,1
n = milligrams of nitric acid formed,
j = milligrams of iron in the igniting coil.
As regards this calculation, it may be pointed out that the various,
corrections indicated above compensate one another partly, so that, for
1 The water-equivalent is the weight of water requiring the same amount of heat
to raise its temperature i° as the calorimetric apparatus (vessel, stirrer, thermometer,
etc.). It is determined by an experiment with a substance of known heat of com-
bustion (e.g., naphthalene, 9692 ; cane sugar, 3957; benzoic acid, 6330 cals.).
The difference between the true calorific power of the substance and that calculated
from the experimental results without taking account of the heat absorbed by the
apparatus, divided by the rise of temperature, gives the required water-equivalent,
if it is not thought desirable to allow for the corrections for the rigorously exact calcu-
lation ; otherwise, a is deduced from equation (i).
The water-equivalent may also be calculated theoretically (with less reliable results)
by multiplying the weights of the different parts of the apparatus by the specific heats
of the materials from which they are made and adding together the products thus
obtained.
A.C,
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